Method for mining soluble ores



June 29, 1954 R wo l- 2,682,396

METHOD FOR MINING SOLUBLE ORES Filed Sept. 17, 1948 4 Sheets-Sheet lATTORNEY June 29, 1954 R. G. HAWORTH 8 METHOD FOR MINING SOLUBLE ORES vFiled Sept. 17, 1948 4 Sheets-Sheet 2 JNVENTOR. Rus s|l G. HclwonhATTORNEY June 29, 1954 R. cs. HAWORTH METHOD FOR MINING SOLUBLE ORESFiled Sept. 17, 1948 4 Sheets-Sheet 3 Q INVENTOR. RussellG. Haworrh BY vWA f: W

' ATTORN EY June 29, 1954 R. G. HAWQRTH 2,682,395

METHOD FOR MINING SOLUBLE ORES Filed Sept. 17, 1948 4 Sheets-Sheet 4 xxx X! x,

i INVENTOR. W RusseHG. Huworfh kg! y ATTORNEY Patented June 29, 1954METHOD FOR MINING SOLUBLE ORES Russell G. Haworth, Carlsbad, N. Mex.,assignor to Potash Company of America, Denver, 0010., a corporation ofColorado Application September 17, 1948, Serial No. 49,833

12 Claims.

This invention relates, generally, to the art of solution mining ofsoluble minerals, and it has particular relation to a novel andeconomical method of solution mining for relatively thin deposits ofsoluble ores, such as ones including soluble potassium salts.

Solution mining i broadly old. However, insofar as I am aware, solutionmining has previously involved operating through holes drilleddownwardly from the ground surface into underground deposits of solubleminerals, such as salt. Although such mining methods are practical forworking mineral deposits of substantial thickness, they are notpractical for working thin deposits (1. e. a few feet thick) located atsubstantial depths, since, in these latter instances, the amount ofmineral that can practically be obtained at the foot of each well orshaft is so small that the cost of driving the wells i prohibitive.Likewise, ordinary mechanical mining methods are not suited to oreconomical for working such thin layer deposits.

According to the present invention, an economical method for workingsub-surface deposits of soluble mineral salts has been provided, whichinvolves a unique combination of mechanical mining methods with a noveltechnique of solution mining. Thus, I provide as by mechanical miningmethods, a main entry down and into such a shallow deposit of solublemineral, and then 7 provide a number of cross or lateral entries leadingoff therefrom at suitable intervals. It is from these cross entries orlaterals that I employ my novel solution mining technique to leach outthe deposit in between adjacent cross entries. As Will appear more fullyhereinafter, the areas of the deposit are leached out in such a way thatbackfill may be introduced into already extracted portions as theleaching progresses in intermediate portions.

An important object of the invention is the provision of an economicalmethod of working subsurface deposits of soluble mineral deposits bysolution mining from underground entries without drilling a plurality ofvertical wells from the surface.

Another important object of the invention is the provision of aneconomical method of working sub-surface deposits of soluble mineraldeposits by a combination of mechanical and solution mining methods, aswell as by solution mining only, and obtaining thereby a reduction inmaintenance and operating cost.

Still another object of the invention is the provision of a method ofoperation from a lateral entry in such a way as to extract or leach outan elongated room in a deposit or bed of soluble mineral, starting witha group of small holes drilled adjacent the top of said deposit andextending for the depth of the room.

Still another object of the invention is the provision of a method ofworking a deposit of soluble mineral from lateral entries therein, whichinvolves leaching out or extracting a plurality of elongated rooms inthe deposit on opposite sides of each lateral entry, leavingintermediate pillars of ore between adjacent rooms, after which therooms may be hydraulically backfilled with sand for support, after whichthe intermediate pillars of ore may be leached out.

Certain other objects of the invention will in part be obvious, and willin part appear hereinafter.

For a more complete understanding of the nature and scope of theinvention, reference may now be had to the following detaileddescription of an illustrative embodiment thereof, taken in connectionwith the accompanying drawings, wherein:

Fig. 1 is a diagrammatic plan view illustrating the manner in which ashallow deposit of soluble mineral ore is worked or mined in accordancewith the present invention;

Fig. 2 is a diagrammatic sectional view taken on line 22 of Fig. 1;

Fig. 3 is a diagrammatic sectional view taken on line 33 of Fig. 1

Figs. 4, 5, 6 and '7 are diagrammatic sectional views taken on line AAof Fig. 1. showing respectively dilferent stages of the solution miningoperation; and

Figs. 8 and 9 are diagrammatic cross sections illustrating a modifiedform of my invention.

Referring now to Fig. l of the drawings, the reference numeral Illdesignates a main entry into a bed of soluble ore l l (i. e. potashdeposits). On centers of 600-650 feet cross entries or lateral entriesl2l2 are cut in the deposit H at right angles to the main entry Ill. Itwill be understood that the dimension of 600-650 feet, although apractical one, is given by way of illustration, and that the inventionis not limited either with respect to this dimension or the otherspecific diproximately thirty feet. Track may be laid in the main entryit and the cross entries l2l2 so as to provide ready access thereto anda means of transporting equipment and men. The entries may each be abouteight feet in height and twelve feet in width.

Although the deposit l I may be nearly horizontal, in most instances itwill have at least a small pitch or dip in one direction or another. Forpurposes of more fully illustrating the invention, it will be assumedthat the deposit i i has an upward pitch from left to right in Figure 1.As will appear presently in detail below, I employ solution miningtechnique in such a way as to leach out or extract from the deposit H aplurality of regularly spaced elongated rooms I l-M which slant upwardlyfrom the lower or left-hand cross entry l2 in Figure 1 and a second setof similar rooms I|5 which slant downwardly from the upper or right-handcross entry 52 in Figure 1. It will also be seen below that, afterformation, the rooms l4l4 and l5l5 are hydraulically filled with sand soas to re-establish bearing support areas therein, and then theintermediate pillars of mineral I i-46 and l'l-ll, respectively, Figurel, are leached out or extracted.

Reference may now be had to Figures 2 and 4-7 for a detailed descriptionof the method of formingthe elongated rooms |5|5. Each of the rooms isstarted by using diamond drills to drill a group of five holes l8l8(Figure 4.) on six foot centers in, or just above, the ore deposit ll toa distance of approximately 300 feet. The first hole i 8 of each groupshould be cored to demine proper vertical elevation. After the holes|llls are drilled, the collar of each hole is reamed to a depth ofapproximately feet and a four inch casing 2e (Figure 2) is placed in thereamed section and cemented in place with concrete 2! to preventleaching in this zone which will remain as a pillar 22, and also servingas a dam to retain brines or solution in the rooms l5l5.

When the casings 2B are in place, a two inch inner pipe 23,approximately 300 feet long, is inserted through each casing into thecorresponding hole l8. Each casing 29 is provided near its outer endwith an inlet port 24 and the end of the casing being sealed to the pipe23, as

at 25. With this arrangement installed, fresh water is introducedthrough each of the ports 24-24 and allowed to trickle along the bottomsof the holes l8l8 to the lower ends thereof, dissolving the soluble ore.

The resulting brine collecting in the bottom or far end of each hole I 8is pumped out through the respective pipes 23 which are connected tosuitable pumping means. This circulation is continued until a slot 26(Figure 5) is out under each hole l8 which extends approximately throughthe thickness of the deposit I l.

The completion of the slots 2626 can be ascertained by a change in thecomposition of the brine withdrawn from the pipe 23. Thus, if thedeposit ll consists of sylvite in a known concentration sandwichedbetween layers of halite 31, a decrease in the K01 content of the brineand a corresponding increase in the NaCl content, will indicate that theslots 2626 have gone through the deposit Ii.

After the slots 2E26 have thus been cut, which may take approximatelyforty-eight hours, they are filled with water and the flow or directionof circulation is reversed. That is, fresh water is v 4 introducedthrough the pipes 23-43 to the bottom of the respective slots 2626,which will fill up with solution and remain filled during leaching. Thesolution of brine is withdrawn from the upper end of each slot 26through the casings 2820 and the ports 2424. The rate of flow isregulated so as to produce a brine saturated with respect to KCl.Dissolution or leaching of the ore takes place largely on the oppositesides of each slot and these become increasingly larger as indicated at2l-2T in Figure 6. Eventually, each group of enlarged holes BI -21 mergeto form a room l5 as shown in Figure 7. At this stage, the only oreremaining in the room 15 will probably be in the floor thereof and thatforming the barrier or pillar 22 at the side of the cross entry l2.

If desired, this ore remaining in the bottom of each room [5 may belargely leached out. To do this, each room is emptied of its contents bypumping it almost dry through the pipe 23 which bends downwardly underits own weight as the cut deepens from its former position in drill holel8 shown in dotted lines in Figure 2. Fresh water is then introducedthrough each of the casings 20-20 and allowed to trickle down the floor,cutting irregular channels therein so as to expose remaining ore whichmay be covered with clay. This water is removed through pipe 23 untilthe irregular channels reach the halite bed 31, when the flow is againreversed so as to refill the room l5 through the pipe 23 and withdrawsaturated brine through the casing 20.

A drop in the K01 concentration in each phase of the leaching operationin the formation of room i5 is usually an indication that the particularphase of my process then in operation should be terminated.

It will be seen that each room IE will be upwards of 36 feet wide whenthe end holes of the group of drill holes [8-4 8 are 30 feet apart. Theholes l8-l8 are spaced apart a distance which is determined to a largeextent by the thickness of the deposit H. Generally speaking, the drillholes should be so spaced in each instance that by the time thecorresponding slots cut through the ore are enlarged to the point ofmerging together as shown in Figure '7, the deposit H will have beenleached through to the bottom.

In the event the overlying bed or root is of salt or other solublematerial, proper spacing of the drill holes will also reduce the amountof soluble waste material extracted with the ore, and this factor shouldbe taken into consideration when the spacing of the holes is determined.

In order that the pillars of ore I'I between adjacent rooms [5 may beleached out without causing the upper surface of the mine to cave in,the rooms 15 are first hydraulically filled with sand, thereby restoringbearing support areas therein. After this operation, the pillars Il maybe leached out as rooms following the technique above described for theextraction of rooms IS.

The rooms M-M are formed in the same general manner as rooms l5-l5, butthe technique is somewhat modified in view of the fact that these rooms'rise or slant upwardly from the lefthand entry I2, Figure 1. For adescription of the method of forming rooms H-M, reference may now be hadto Figure 3.

For each room M, a group of say five drill holes are drilledcorresponding to those drilled for each room I5. The collar of eachdrill hole is reamed and a four inch casing 30 is cemented in place soas to leave a barrier or dam of ore 3! thereunder. A 300-foot length ofpipe 32 is then inserted in each drill hole and slots corresponding toslots 2626, Figure 5, are cut by circulating fresh water through thepipes 32 to the upper end of each drill hole and allowing it totrickleback along the bottom thereof to a hose 33 inserted through the casing30 resting on the bottom of the slot.

During this operation the brine or water, which may be either hot orcold, should be held at the lowest practical limit by pumping orsiphoning through the hose 33. This tends to prevent saturation of theliquid by the walls of the slot, and concentrates the dissolving actionon the bottom of the slot, thus increasing the rate of slot formation. I

When the slot has been formed to the desired depth, I prefer to removethe hose 33 and to connect the pipes 30 and 32 in a manner similar topipes 20 and 23 (Figure 2). Fresh water or unsaturated brine may then beintroduced through pipe 30 and circulated through the slot in contactwith the walls at a rate which will substantially saturate the liquidwith ore. The slot is therefore filled with liquid under some pressure,and is exhausted through pipe 32 at the desired rate. Obviously thewalls expose the greatest area to the action of the liquid, and furnishthe greatest majority of the soluble material required to saturate theliquid.

When the concentration of the brine begins to drop, it may be desirableto empty the slot or hole and reverse the flow of fresh water so as toallow it to trickle down the bottom of the hole to the hose 33, whichmay be reinserted for the purpose, so as to expose more ore, or in someinstances the water may be exhausted through the pipe 30. Thereafter,the hole may again be filled and additional ore leached out until eachroom [4 is completed. Then each room I4 may be hydraulically filled withsand and the intermediate pillars !6 leached out.

The saturated brine withdrawn during the leaching operations may bepumped to a surface refining plant designed to extract the valuablesalts, e. g. KCl, therefrom in known manner.

In places where the deposit H is horizontal, the ore may be leached outeither according to the method described above for forming rooms -!5, orby that for forming rooms I4|4. If desired, a combination of bothmethods can be used.

In certain instances, the main entry Ill and the cross entries l2l2 maybe more economically formed by following the techniques described abovefor the formation of rooms |5l5. The resulting elongated entries wouldrequire only a minimum amount of trimming by usual mechanical miningmethods to make them suitable for haulage entries.

The invention hereinfore described course, particularly advantageous inmining thin beds of water soluble ore, which usually consist principallyof the various salts or salt'combinations of the alkali metals. From acost standpoint, the comparison in cost between my method of mining andthe conventional mechanical methods of mining will be most marked whenemployed in the mining of beds having a height or depth of less thanapproximately six feet, but is oftentimes cheaper than mining methodseven I in beds twelve feet or more in depth.

In most instances it will be found preferable to employ the systemhereinbefore described which incorporates the forming of a slot byproper manipulation of the liquid prior to theis, of

6 major solution mining operation. For example, beds of sylvinite areoften found between beds of salt and it is obviously desirable tominimize the solution of the salt to the greatest possible extent.Forming a vertical slot through a bed, say four feet thick and threehundred feet long will expose twelve-hundred square feet of ore on eachside of the out, while the top of say a two inch slot will expose onlyfifty square feet of the salts in the overlying bed. The bottom of theslot is usually filled with slimes and does not present a seriousproblem. Because of the large area exposed, solution proceeds at arelatively rapid. rate, particularly when compared to a vertical hole ofsay six inch diameter, which would expose an areaof only approximatelysix square feet in a four foot bed.

It is not in each instance necessary that the drill holes be disposed ator near the level of the overhang bed, since in some instances it willbe advantageous to form these openings along the line midway between theupper and lower beds, thus as shown in Figures 8 and 9, I may form aplurality of spaced drill holes t2 in a bed 43 of soluble ore disposedin upper and lower beds 44 and 46 respectively. Hot or cold water orunsaturated brine may be pumped through these holes, preferably underpressure, until the walls have dissolved to the extent necessary to formenlarged interconnected cavities 41 indicated at an intermediate stagein Figure 9. This system might be utilized, for example, where the upperand lower beds were of a substantially water-soluble material.

It should also be understood that in many instances cross entries,drifts, and the like may be formed by properly utilizing my miningmethod at a lower cost than the same operation could be formed bymechanical mining methods, this being true, of course, only in instanceswhere the operations are to be formed in water-soluble deposits such ashalite.

Since certain further changes or modifications may be made in theforegoing methods and different embodiments of the invention may bepracticed without departing from the spirit and scope thereof, allmatter described above in connection with the accompanying drawings, isintended as illustrative and not in a limiting sense.

I claim:

l. The method of mining an inclined watersoluble mineral deposit fromtwo parallel, widely spaced, lateral entries therein, which comprises,drilling from the upper entry a plurality of spaced groups of parallelsmall diameter drill holes downwardly from said upper entry toapproximately one-half the distance between said entries, and on a planeadjacent to the top of said deposit, inserting a pipe into each drillhole to approximately the remote end thereof, introducing fresh waterinto said hole on the outside of said pipe and withdrawing the resultingbrine from the bottom of said hole through said pipe and continuing suchcirculation until a slot is cut underneath each drill hole to a levelnear the bottom of said deposit, filling each slot with water,introducing unsaturated water through said pipes into the bottom of eachslot and withdrawing saturated brine from the upper end thereof after ithas circulated along the bottom of the slot thereby dissolving away thesides and bottom of each slot sufiiciently to leach away the oreseparating adjacent slots so as to form for each group of drill holes anelongated room in said deposit approximately equal in width to the widthof said group of drill holes, drilling a plurality of spaced groups ofparallel small diameter drill holes upwardly from the lower entry toapproximately half the distance between said entries and on a planeadjacent to the top of said deposit, inserting a pipe into each of saidupward drill holes approximately to the remote end thereof, introducingfresh water into the upper end of each upward drill hole through saidpipe therein and allowing it to run back down the bottom of said holeand continuing such circulation until a slot is cut underneath eachdrill hole nearly through to the bottom of said deposit, filling eachsaid slot with water, introducing unsaturated Water into the lower endof each said slot and withdrawing saturated brine from the upper endthereof through the pipe therein after it has circulated along thebottom of each said slot thereby dissolving away the sides and bottomthereoi suificiently to leach away the ore separating adjacent slots soas to form for each group of said drill holes from said lower entry anupwardly sloping elongated room in said deposit approximately equal inwidth to the width of the corresponding group of drill holes drilledfrom said lower entry, said drill holes drilled from said upper andlower entries being so spaced apart relative to the thickness of saiddeposit that by the time said adjacent elongated slots have becomeinterconnected into an elongated room said deposit will be approximatelyleached through to the bottom thereof.

2. The method called for in claim 1 wherein said elongated rooms arehydraulically filled in with sand to restore bearing support areastherein, and the ore remaining between said adjacent rooms is thenleached out.

3. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a lateral entry extendingpast a portion of said mineral deposit for a substantial distance,drilling a plurality of holes near the top of said deposit extendingfrom said entry in a substantiall parallel arrangement following thecontour of said deposit and penetrating same for a substantial distance,introducing water into said drill holes throughout their lengthwiseextent through an inlet conduit directed lengthwise of said drill holestoward the remote end thereof so as to progressively produce verticalslots by dissolution extending substantially throughout the depth ofsaid deposit, withdrawing solution formed by dissolution of solublematerial in said slot-forming operation at a point near, but spacedfrom, the entry end of said drill holes, and then introducing additionalwater into the slotted area at a high point therein, and simultaneouslywithdrawing the resulting solution from a low point therein so as toleach out the soluble material between said slots and thereby form asingle room in said deposit within the drilled area.

4. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a lateral entry extendingpast a portion of said mineral deposit for a substantial distance,drilling a plurality of holes near the top of said deposit extendingfrom said entry in a substantially parallel arrangement following thecontour of said deposit and penetrating same for a substantial distanceat an elevation above the bottom of the deposit, introducing water intosaid drill holes throughout their lengthwise extent through an inletconduit directed lengthwise of said drill holes toward the remote endthereof so as to progressively produce vertical slots by dissolutionextending substantially throughout the depth of said deposit,withdrawing solution formed by dissolution of soluble material in saidslot-forming operation at a point near, but spaced from, the entry endof said drill holes, and then introducing additional Water into theslotted area at a high point therein, and simultaneously withdrawing theresulting solution from a low point therein promptly after its formationso as to leach out the soluble material between said slots and therebyform' a single room in said deposit within the drilled area.

5. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a lateral entry extendingpast a portion of said mineral deposit for a substantial distance,drilling a plurality of horizontally ranging holes near the top of saiddeposit extending from said entry in a substantially parallelarrangement following the contour of said deposit and penetrating samefor a substantial distance, introducing water into said drill holesthroughout their lengthwiv e extent through an inlet conduit directedlengthwise of said drill hole toward the remote end thereof so as toprogressively produce vertical slots by dissolution extendingsubstantially throughout the depth of said deposit, then filling saidslots with water introduced through a conduit having its outlet near ahigh point in each slot so as to leach out the remaining portions ofsaid drilled deposit, and simultaneously withdrawing the resultingsolution through another conduit having its intake at a low pointtherein, promptly after formation of said solu tion so as to preventdissolution of material from the underlying soluble layer.

6. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a lateral entry extendingpast a portion of said mineral deposit for a substantial distance,drilling a plurality of horizontally ranging holes near the top of saiddeposit extending from said entry in a substantially parallelarrangement following the contour of said deposit and penetrating samefor a substantial distance, sealing the deposits around said holes fromsaid entry for a short distance interiorly thereof to preventdissolution thereof, introducing water into said drill holes throughouttheir lengthwise extent through an inlet conduit directed lengthwise ofsaid drill hole toward the remote end thereof so as to produce verticalslots by dissolution extending substantially throughout the depth ofsaid deposit, then filling said slots with water introduced through aconduit having its outlet near a high point in each slot so as to leachout the remaining portions of said drilled deposit, and simultaneouslywithdrawing the resulting solution through another conduit having itsintake at a low point therein, promptly after forrna tion of saidsolution so as to prevent dissolution of material from the underlyingsoluble layer.

'7. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a lateral entry extendingpast a portion of said mineral deposit for a substantial distance,drilling a plurality of horizontally ranging holes near the top of saiddeposit extending from said entry in a substantially parallelarrangement following the contour of said deposit and penetrating samefor a substantial distance, introducing water into said drill holesthroughout their lengthwise extent through an inlet conduit directedlengthwise of said drill hole toward the remote end thereof so as toproduce vertical slots by dissolution extending substantially throughoutthe depth of said deposit, then filling said slots with water introducedthrough a conduit having its outlet near a high point in each slot so asto leach out the remaining portions of said drilled deposit, said drillholes being so spaced apart relative to the thickness of said depositthat by the time said adjacent slots have becomeinterconnected. to forma single room the drilled portion of said deposit will be substantiallyleached through to the bottom thereof, and simultaneously withdrawingthe resulting solution through another conduit having its intake at alow point therein, promptly after formation of said solution so as toprevent dissolution of material from the underlying soluble layer.

8. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition which dip from the horizontal, which comprisesforming a lateral entry extending past a high portion of said mineraldeposit for a substantial distance, drilling a plurality of holesextending downwardly from said entry to a substantial distance into saiddeposit on a plane adjacent the top thereof, discharging water into saidholes through a conduit having its outlet adjacent the upper endthereof, withdrawing resulting solution through another conduit havingits intake at a low point in the deposit adjacent the lower end of eachhole, and continuing the introduction of water at said upper end untilsubstantially all the soluble content within the drilled area has beendissolved.

9. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition which rise from the horizontal, which comprisesforming a lateral entry extending past a low portion of said mineraldeposit for a substantial distance, drilling a plurality of holesextending upwardly from said entry to a substantial distance into saiddeposit on a plane adjacent the top thereof, discharging water into saidholes through a conduit having its outlet adjacent the upper endthereof, withdrawing resulting solution through another conduit havingits intake at a low point in the deposit adjacent the lower end of eachhole, and continuing the introduction of water at said upper end untilsubstantially all the soluble content within the drilled area has beendissolved.

10. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material of dif- 6ferent composition, which comprises forming a small opening into such asoluble deposit from a breast near the upper boundary of the bed andapproximately parallel thereto, circulating water through a conduitextending through the opening and having its outlet at the end thereofremote from the breast, removing resulting solution from another conduithaving its intake at a point near the breast in an action in which avertically ranging slot is formed in the deposit, and then reversing thecirculation of water into the opening through said conduits so as toleach out soluble material of the deposit from the area adjoining saidslot.

11. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a small opening into sucha soluble deposit from a breast near the upper boundary of the bed andapproximately parallel thereto, circulating water through a conduitextending through the opening and having its outlet at the end thereofremote from the breast, removing resulting solution from another conduithaving its intake at a point near the breast in an action in which avertically ranging slot is formed in the deposit, reversing thecirculation of Water into the opening through said conduits so as toleach out soluble material of the deposit from the area adjoining saidslot, and finally reversing the circulation through said conduits asecond time when the dissolved content drops below a predeterminedminimum value.

12. The method of mining a water-soluble mineral from an undergrounddeposit lying between layers of other water-soluble material ofdifferent composition, which comprises forming a small opening into sucha soluble deposit from a breast near the upper boundary of the bed andapproximately parallel thereto, circulating water through a conduitextending through the opening and having its outlet at the end thereofremote from the breast, removing resulting solution from another conduithaving its intake at a point near the breast in an action in which avertically ranging slot is formed in the deposit, and thereafterperiodically reversing the direction of circulation through saidconduits as the solubility of the resulting solution falls below apredetermined minimum value.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 508,523 Mitchell Nov. 14, 1893 2,161,800 Cross June 13, 19392,200,665 Bolton May 14, 1940 2,365,591 Ranney Dec. 19, 1944 FOREIGNPATENTS Number Country Date 89,840 Austria Oct. 25, 1922 135,722 GermanyNov. 4, 1902 235,723 Germany June 20, 1911 445,977 Germany June 20, 1927

